Photoresponsive semiconductor alloy materials and devices constructed using thin film technology have been extensively investigated in the last two decades. Recently, considerable efforts have been made to develop systems for depositing polycrystalline, microcrystalline and amorphous semiconductor alloy materials, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n and n-i-p type photovoltaic devices which are, in operation, substantially equivalent to their crystalline counterparts.
It is now possible to prepare amorphous silicon alloy materials by glow discharge or other vacuum deposition techniques, said materials possessing (1) acceptable concentrations (less than 10.sup.16 per cubic centimeter per eV) of localized defect states in the energy gaps thereof, and (2) high quality electrical and optical properties. Such amorphous materials and the associated deposition techniques are fully described in U.S. Pat. No. 4,226,898, entitled Amorphous Semiconductors Equivalent To Crystalline Semiconductors, issued in the names of Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980; U.S. Pat. No. 4,217,374, issued in the names of Stanford R. Ovshinsky and Masatsugu Izu on Aug. 12, 1980, also entitled Amorphous Semiconductors Equivalent To Crystalline Semiconductors; and U.S. Pat. No. 4,517,223, issued in the names of Stanford R. Ovshinsky, David D. Allred, Lee Walter, and Stephen J. Hudgens on May 14, 1985 and entitled Method Of Making Amorphous Semiconductor Alloys And Devices Using Microwave Energy. As disclosed in these patents, all of which are assigned to the assignee of the instant invention and the disclosures of which are incorporated by reference, fluorine introduced into the layers of amorphous silicon alloy material or amorphous silicon:germanium alloy material operates to substantially reduce the density of the localized defect states in the energy gap thereof and facilitates the addition of other alloying, compensating and doping elements.
It is also now known that photovoltaic device efficiency can be further enhanced by stacking multiple photovoltaic cells. More particularly, when these cells are arranged in serial optical and electrical fashion, each stacked cell is fabricated with a different band gap. This is accomplished by employing a photogenerative layer in each stacked cell characterized by a particular semiconductor alloy material in an effort to more efficiently collect therein only specific portions of the entire spectrum of available light incident upon the stack of cells. Specifically, such stacked structures can be fabricated so that in the first of the stacked cells a relatively wide band gap semiconductor alloy material, such as a silicon:carbon alloy material, absorbs only the shorter wavelength light while subsequent cells are fabricated of smaller band gap semiconductor alloy materials, such as a silicon alloy material and silicon:germanium alloy materials, so as to absorb the longer wavelengths of light which pass through the first cell. Such stacked cells not only include a plurality of successively deposited layers of semiconductor alloy material, but also preferably include a back reflector for increasing the percentage of incident light reflected from the substrate back through those layers of semiconductor alloy material from which the stacked cells are fabricated.
Through the inventive efforts of the assignee of the subject invention, it is now possible to successively deposit layers of semiconductor alloy material onto a large area substrate, such as a continuously moving web of substrate material. This type of large area deposition of successive layers of semiconductor alloy material of varying band gaps represents a deposition technique developed by the assignee of the subject invention, the applicability of which with respect to the solid state light emitting elements, light detecting elements and solid state photonic circuitry of the subject invention will be demonstrated in subsequent paragraphs of this background section of the instant specification.
It is also important to note that considerable efforts are also being made to fabricate related semiconductor devices which can be employed in the construction of integrated, solid state electro-optical circuits, the electronic components of which are fabricated of non-crystalline semiconductor alloy materials, such as amorphous silicon, amorphous germanium and amorphous silicon-germanium alloys. P-i-n type amorphous silicon:hydrogen alloy devices have been known since the work of Carlson at RCA in 1975, as described in U.S. Pat. No. 4,064,521, and the possibility that such amorphous silicon:hydrogen alloy and amorphous silicon:carbon alloy p-i-n type diode structures could be made light emitting was actually suggested as early as 1979 by Pankove, also working at RCA.
Such light emitting diodes operate to emit light due to the recombination of electrons from the conduction band and holes from the valence band which meet in the band gap of the particular semiconductor alloy material under consideration. Hence the width of the band gap of that particular semiconductor alloy material determines the wavelength of the light generated by the diode. However, because of the band gap structure of silicon:hydrogen alloys and germanium:hydrogen alloys, the recombination of electrons and holes in the band gap of these alloys at room temperature generates predominantly heat or photons in the infra-red region of the solar spectrum. Group III-V semiconductors such as gallium arsenide or indium phosphide have also been previously employed for the purpose of emitting infra-red photons. Further, wider band gap single crystalline semiconductors such as gallium photphide or gallium-aluminum arsenide have been previously employed for emitting visible wavelengths of light.
It has further been found possible to link such solid state light emitting devices with light detecting devices, also in solid state crystalline format, for developing cross-communication therebetween. Such light detecting devices also rely on transitions occurring across the band gap of similar semiconductor materials, but the transitions are the reverse of the ones described for light emitters. More particularly, when a photon of incident radiation impinges on a semiconductor material, it excites an electron and creates a hole, provided of course that the photon energy is greater than the band gap of the semiconductor material. If, in addition to the incident radiation, a potential is placed across the semiconductor material, the photogenerated electrons and holes are forced to move in opposite directions and generate a photo-induced current which can be amplified and recorded. While such light detecting devices can be built from a uniform single layer of semiconductor material, in the crystalline world a p-n type junction is preferably employed. In this manner, a built-in electric field is incorporated about the junction region so as to aid in the collection of said excited electron-hole pairs. As a matter of fact, "avalanche photodiodes" have been constructed in which electrons and holes moving through a semiconductor material under the influence of a high electric field actually generate additional electron-hole pairs so as to amplify the electrical signal to be detected.
When fabricating amorphous light emitting diode structures, a plurality of very thin layers of thin film semiconductor alloy material of differing composition and conductivity-types are successively deposited so as to obtain the necessary electrical and optical properties to effectuate light emission in said structures. More particularly, successively deposited atop a substrate are layers of n-doped semiconductor alloy material, intrinsic semiconductor alloy material and p-doped semiconductor alloy material so as to provide a p-i-n diode. In order to emit light when such a diode is forward biased, it is necessary that holes from the p-doped layer of semiconductor alloy material and electrons from the n-doped layer of semiconductor alloy material be provided with efficient access to the intrinsic layer of semiconductor alloy material interposed therebetween. This access can be facilitated by aligning the conduction bands of the intrinsic and n-doped layers at the intrinsic n-doped interface for the efficient movement of electrons into the intrinsic layer and by aligning the valance bands of the intrinsic and p-doped layers at the intrinsic p-doped interface for the efficient movement of holes into the intrinsic layer. However, the intrinsic layer of semiconductor alloy material typically has a different band gap than that of either of the doped layers of semiconductor alloy material and, consequently, misalignment of the valence and conduction bands at the respective interfaces therebetween occurs. For instance, heretofore scientists have employed a p-doped amorphous silicon carbon hydrogen alloy layer and an n-doped amorphous silicon carbon hydrogen alloy layer with an intrinsic amorphous silicon carbon hydrogen alloy layer disposed therebetween in an attempt to match the corresponding valance and conduction bands of the layers at their respective interfaces. While this choice of materials caused the conduction bands at the n-doped and intrinsic layer interface to be aligned, the valence bands at the p-doped and intrinsic layer interface were misaligned. The result was the inefficient transfer of holes into the intrinsic layer with a correspondingly inefficient emission of light from the diode. It is therefore a first objective of the subject invention to align the valence bands at the p-doped: intrinsic layer interface as well as the conduction bands at the n-doped: intrinsic layer interface so as to provide a diode structure fabricated from amorphous semiconductor alloy material, which structure is characterized by the efficient generation and emission of photons of visible light.
As should be apparent from the foregoing discussion, the fabrication of light emitting structures, even if constructed as crystalline p-n diodes, is a very complex task which has heretofore been dependent upon the epitaxial growth of carefully lattice matched crystalline semiconductor materials. While, as also mentioned hereinabove, Pankove had suggested the ability of amorphous semiconductor alloy materials to function in a light emitting sense as early as 1979, until the subject invention, as detailed herein, scientists have struggled to fabricate visible light emitting diodes from amorphous semiconductor alloy materials. It is therefore another important object of the present invention to fabricate efficient light emitting diodes from amorphous semiconductor alloy materials generally and efficient light emitting diodes from amorphous silicon:carbon alloy materials specifically, which diodes include a layer of p-doped microcrystalline silicon alloy material. Further, by employing the fabrication techniques developed by the assignee of the subject invention, alluded to hereinabove, uniformity and homogeneity of micron-scale light emitting diodes is made possible.
One of the most important potential applications of such solid state, thin film light emitting diodes is as one of the active elements in a photonic circuit. As used herein, the term "photonic circuit" is defined as a circuit in which light rather than electricity is employed to generate, transmit, receive or process signals comprised of photons. A photonic circuit must not only be capable of converting electrical signals into light at the transmitting source of the signals, but must also be capable of converting the light back into electrical signals at the receiving end thereof. The inventors of the subject matter disclosed herein propose to integrate the previously detailed light emitting diodes fabricated from thin film semiconductor alloy materials along with solid state light detecting elements also fabricated from thin film semiconductor alloy materials, said light detecting elements adapted to convert the light generated by the light emitting elements into electronic pulses thereby providing photonic communication therebetween. In previously filed U.S. patent application Ser. No. 886,287 assigned to the assignee of the subject application, the inventors therein disclosed the use of non-linear, optically responsive semiconductor alloy material from which to fabricate photonic logic gates, said logic gates adapted to optically model the properties of the logical connectives AND, OR and NOT. In this manner, it was suggested that the disclosed photonic logic gates and specially fabricated thin film transistors could be employed as the basic building blocks of a photonic supercomputer. In line therewith, a second essential objective of the subject disclosure is the fabrication of a thin film, solid state light emitting diode which is operatively disposed in intercommunicative relationship with an integrated thin film, solid state light detecting diode so as to fabricate photonic circuitry which could be utilized as logic elements or as otherwise desired.
Such solid state, thin film, integrated electro-optical circuit structures may be fabricated in three dimensional format and are preferably constructed using multiple stacked layers of thin film materials including amorphous semiconductor alloy materials, metallic conductors and insulating materials deposited on a large area substrate such as the elongated web of substrate material alluded to hereinabove. As the number or the density of discrete solid state elements in such integrated circuit structures is increased, the efficient, economical and reliable interconnections between solid state elements located on vertically and/or spaced planes of the structure and the intercommunication between those solid state elements and electronic components located externally to the structure become increasingly important and exponentially complex. One aspect of such interconnective or intercommunicative paths, particularly in high density three dimensional structures, is electrical noise or cross-talk which can be generated between discrete solid state elements and conductors operatively disposed on the same plane, operatively disposed on different planes of the structure, or operatively disposed on conductors interconnecting the integrated structure to an external circuit element. It is yet a further objective of the subject invention to provide solid state, integrated deposition and fabrication techniques which provide interconnective and intercommunicative paths exhibiting inherently low susceptibility to electrical noise or cross-talk for minimizing potential signal attenuation.
One of the distinct advantages, as discussed hereinabove, of utilizing discrete, thin film, solid state elements and thin film, solid state integrated electronic and photonic structures is that they may be fabricated in large areas, thus making possible the implementation of very large scale integration (VLSI) and ultra-large scale integration (ULSI) electronic circuitry, without the use of costly micron or submicron geometries. Larger and much less expensive feature sizes, such as 5 microns to 20 microns or more may be used, since said layers of thin film semiconductor material may be reliably deposited and patterned over very large areas, with greater ease of fabrication and higher yields than their crystalline counterparts. Also, unlike crystalline silicon devices which must be epitaxially grown upon a crystalline silicon substrate, thereby enhancing the difficulty of building multiple layers of circuitry, thin film semiconductor alloy material may be deposited in multiple vertically separated layers or planes, thereby yielding truly three dimensional structures. Accordingly, the assignee of the present invention is developing ultra-large capacity three dimensional thin film amorphous semiconductor memories and the necessary technology to construct a fully integrated thin film central processing unit or computer which is not only fabricated from the amorphous semiconductor alloy materials, but which also employs the photonic concepts disclosed herein.
Finally, there has been a great deal of recent interest in the development of large area image scanning devices wherein amorphous semiconductor alloy material is utilized to fabricate the solid state elements which are capable of detecting the intensity of light reflected from an image-bearing document and generating an electrical signal responsive thereto. Those scientists engaged in such image scanning device development recognize the fact that one important technique for reducing the costs of large area image scanning devices is the simultaneous fabrication of the light emitting diode along with the light detecting device, such as a photodiode, on a common, large area substrate so that the light source would become an integrated part of the large area array of photosensitive elements. Not only would such integrated fabrication decrease manufacturing costs, but the light source would be fixed in operative disposition relative to the light detecting diodes as well as the image bearing document so as to optimize the intensity of image illumination and the reflection of said illumination onto the associated light sensitive array of solid state elements. Further, by employing the material design capabilities of the assignee of the subject invention, it is possible to fabricate photogenerative layers of semiconductor alloy material which are specifically tailored to (1) emit only desired wavelengths of radiation and (2) respond to those specific wavelengths of radiation generated by the light emitting element. It therefore is yet another object of the subject invention to fabricate discrete, integrated, solid state light emitting elements and light detecting elements through the vapor deposition of successive layers of semiconductor alloy material onto large area substrates.
Therefore, Applicants believe that through the utilization of the principles outlined herein, electro-optic technology and photonic circuit design has matured to the point at which it is now possible to forsee the evolution of optical integrated circuits that will require no electronics whatsoever. In the meantime, the integrated light emitting elements and light detecting elements of the subject invention will provide for the fabrication of a solid state, thin film, large area array of discrete, interconnected light sensitive elements which are readily adaptable for use in image scanning devices.
These as well as other objects, advantages and subsidiary capabilities of the subject invention will become more apparent from the detailed description of the drawings, the claims and the multiple figures which are disclosed in the following paragraphs.